The Whau River, despite its name, is actually an estuarial arm of the southwestern Waitemata Harbour, within the Auckland metropolitan area in New Zealand. It extends south for five kilometres from its mouth between the Te Atatu peninsula and the long thin Rosebank Peninsula in Avondale.

The river extends past the suburbs of Glendene and Kelston, between Auckland city to the east and Waitakere city to the west. It has one small estuarial tributary arm, the Wairau Creek in the southwest.

The area around the mouth of the river is protected as the Motu Manawa/Pollen Island Marine Reserve. Pollen Island (Motu Manawa in Maori) and the nearby small Traherne Island, are located at the northern tip of the peninsula. Traherne Island is connected to the mainland by causeways and bridges that are part of State Highway 16.